An IMEI is an “electronic serial number” that consists of 15 digits, is in one-to-one correspondence with each mobile station (MS), and is unique in the world. Each mobile station is given a globally unique IMEI after being assembled. During a process from manufacturing to delivery for service, a corresponding IMEI of each mobile station is recorded by a manufacturer that performs the manufacturing. In a communications system, only a base station controller (BSC) can deliver an identify request (Identify Request) message to the mobile station to require the IMEI.
Both the mobile station and a serving GPRS support node (SGSN) include an unnumbered information (UI) frame transmission state variable V(U) used for recording a sequence number of a next UI frame to be sent, and a range of the sequence number is 0-511. Each time a UI frame is sent, the V(U) is added by 1. Each UI frame carries a frame sequence number N(U), and a value of the N(U) equals a value of the V(U) when the UI frame is sent.
However, when the base station controller actively delivers the identify request message to query the IMEI of the mobile station, N(U) inconsistency between the mobile station and the serving GPRS support node may be caused. Therefore, a compatibility problem is generated between the mobile station and the serving GPRS support node, and the mobile station may even be offline.